Field of the Invention
The present invention relates to image analysis that determines a positional relationship in a predetermined direction between two three-dimensional images.
Description of the Related Art
In comparison and interpretation of a plurality of three-dimensional images each comprising a plurality of tomographic images as slices obtained perpendicularly to a body axis direction, a technique for displaying corresponding ones of the tomographic images in parallel has been known (Japanese Unexamined Patent Publication No. 8(1996)-294485, for example). In this technique, correspondences between the tomographic images are determined based on a pair of corresponding slice positions specified by an operator and on slice thickness of the respective three-dimensional images, for example. Based on the correspondences, the corresponding tomographic images are displayed in parallel. As a method for automatically determining the pair of corresponding slice positions proposed in Japanese Unexamined Patent Publication No. 8(1996)-294485, a correlation operation is carried out between the three-dimensional images to be compared and interpreted, and the corresponding positions are determined by finding an amount of shift between the images in the predetermined structural axis direction at the time the correlation becomes highest. Target regions on which the correlation operation is carried out (hereinafter referred to as ROIs) are selected by the operator or automatically selected through frequency analysis or the like.
In the case where axial images constituting a plurality of three-dimensional images representing a human body are compared and interpreted, physicians carrying out actual image interpretation wish to display corresponding ones of axial images in parallel by determining correspondences between the axial images with respect to vertebral bones that are not easily affected by respiration or the like.
However, at the time of finding correspondences in a body axis direction by a correlation operation between three-dimensional images as targets of comparison and interpretation, if regions targeted by the correlation operation include a structure such as organs or the like other than vertebral bones, the texture of the vertebral bones or the like having periodicity in the body axis direction contributes less to the correlation between the three-dimensional images due to the periodicity. Therefore, the correspondences are actually determined with respect to a structure other than the vertebral bones, and axial images having a wrong positional relationship between the vertebral bones may be displayed in some cases. FIG. 20 schematically shows an example of such a case. For example, in the case where a tomographic image having a high correspondence with a tomographic image cut by a cross-sectional plane near the center of a vertebral bone B2 in a three-dimensional image V1 is identified in a three-dimensional image V2 while attention is paid to a whole of each of the tomographic images, the texture of liver changes greatly depending on a position of a cross-sectional plane of each of the tomographic images along a body axis direction obtained from the three-dimensional image V2. However, the texture of the vertebral bones does not change so much as the liver. Therefore, a tomographic image having high similarity in the texture of the liver is identified in the three-dimensional image V2. In reality, since a positional relationship between the liver and the vertebral bones can change in tomographic images of chest and abdomen of a subject due to respiration or the like, the position of the cross-sectional plane of the tomographic image identified in the three-dimensional image V2 does not necessarily represent a position near the center of the vertebral bone B2 in the three-dimensional image V1.
On the other hand, in the case where a target of correlation operation is limited to regions of vertebral bones alone, different vertebral bones may be related to each other due to the periodicity of the texture of the vertebral bones. Consequently, axial images wherein vertebral bones are not aligned to each other are displayed in parallel in some cases. FIG. 21 schematically shows an example of such a case. In the case where a tomographic image having similar vertebral bone texture to a tomographic image cut by a cross-sectional plane near the center of a vertebral bone B2 in a three-dimensional image V1 is identified in a three-dimensional image V2, not only an image near the center of the bone B2 but also a tomographic image cut by a cross-sectional plane near the center of a vertebral bone B1 in the three-dimensional image V2 are similar to the tomographic image cut by the cross-sectional plane near the center of the vertebral bone B2 in the three-dimensional image V1, due to the periodicity of the vertebral bone texture. Therefore, the correlation of the tomographic image cut by the cross-sectional plane near the center of the vertebral bone B2 in the three-dimensional image V1 with the tomographic image cut by the cross-sectional plane near the center of the vertebral bone B1 in the three-dimensional image V2 becomes higher than the correlation with the tomographic image cut by the cross-sectional plane near the center of the vertebral bone B2 in the three-dimensional image V2, due to an effect by a difference in imaging conditions or the like between the two three-dimensional images. As a result, the vertebral bone B2 in the three-dimensional image V1 can be related to the vertebral bone B1 in the three-dimensional image V2.